WASHINGTON, D.C. -- Parents can have confidence about the safety of the standard childhood vaccine schedule, according to a new report from an expert panel.

The Institute of Medicine, which advises the government on health, looked into the issue due to concerns from some parents that children today receive too many vaccines, too soon.

But delaying shots only prolongs the time that babies and children are vulnerable to "devastating diseases," says co-author Pauline Thomas, an associate professor of preventive medicine at New Jersey Medical School.

"There is ample evidence that it's not safe not to follow the schedule," Thomas says.

"It's well known that in places where vaccines are delayed or missed, that's where we are beginning to see vaccine-preventable disease outbreaks."

Although the majority of doctors stand firmly behind vaccination, the issue is hotly debated among parents, particularly those too young to remember scourges like measles, polio and whooping cough. To address parents' concerns, the Institute of Medicine has conducted more than 60 studies of vaccine safety since the 1970s. Some parents say the vaccine schedule is too crowded.

Under the recommended plan from the Centers for Disease Control and Prevention, children today are vaccinated against 14 infectious diseases, receiving up to 24 vaccines by their second birthday, and up to five in one office visit. Julie Punishill, a Connecticut mother of two, said she spaces out her children's vaccines whenever possible, instead of following the CDC's advice to give her kids multiple shots at once.

Punishill says she was frightened by her nephew's reaction to a combination shot, after which he spiked a high fever for several days, became very cranky and didn't sleep much. She sees spacing out vaccines as a compromise with her husband, who didn't want to vaccinate at all.

"Neither of my girls had severe reactions," Punishill says.

"They would more often than not have a 24-hour period of either being more sleepy or restless, mild temperature and crankiness." Punishill says she would like scientists to continue to study the vaccine schedule.

"Both my girls were completely vaccinated with everything they needed by the time they entered kindergarten," Punishill says.

While earlier reports from the Institutes of Medicine largely focused on the safety of individual vaccines, this is the first to take a comprehensive look at the safety of the entire schedule, Thomas says.

Although 90% of children are fully vaccinated by the time they enter kindergarten, up to 40% of parents skip or space out some of their children's shots, following alternative schedules due to concerns over safety and side effects, studies show.

While some parents worry about individual vaccines, others wonder about "overwhelming" the immune system, or the cumulative effect of getting so many shots. Fewer than 1% of children receive no vaccines, the report says.

Perhaps the most popular alternative schedule is found in The Vaccine Book, by pediatrician Robert Sears, which spaces out vaccines.

"I created an alternative schedule because a growing number of families are refusing vaccines, simply because they don't like the overload of so many shots together when their babies are so young," Sears said.

"Instead of only offering parents an all-or-nothing approach to vaccines -- either get all the shots on the regular schedule or get nothing -- I believe that more parents will feel comfortable with vaccinating if they can do it more gradually. So, I give the most important vaccines first -- whooping cough, meningitis, rotavirus -- so that babies have protection from the riskiest diseases, but I spread the dosing out a bit. I delay vaccines that protect against diseases that American babies aren't immediately at risk of catching," such as hepatitis B and polio, he says.

Sears said the new report "really does nothing to boost parents' confidence in the full vaccine schedule, and I believe that alternative vaccine schedules will continue to be a popular choice.

Vaccine researchers Peter Hotez, the father of an autistic child, rejects Sears' schedule, noting that it has never been put to a scientific test.

The CDC schedules the timing and doses of childhood shots through careful scientific testing, to optimize children's immune response and protect them during the years when they are most vulnerable, says Hotez, who wasn't involved in the new report.

"The concept that you are going to overload a child's immune system by giving too many vaccines at once makes no sense," says Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine.

Daily life - from crawling around the floor to petting a dog - exposes babies to far more antigens, the proteins that stimulate the body to produce antibodies against diseases, than vaccines. Babies encounter billions of microbes simply leaving the birth canal, Thomas says.

Paul Offit, who developed a vaccine against rotavirus, says the Food and Drug Administration requires manufacturers to test new vaccines in combination with previously approved shots, to make sure they are safe and effective. Contrary to online rumors, "the notion that vaccines aren't tested in combination is wrong," Offit says.

Thomas says she hopes the report will reassure parents and encourage them to fully protect their children.

"It makes no sense to alter the schedule, and in fact it is even dangerous," Hotez says. "When you play with vaccine schedules, you are playing with fire."